This invention relates to an amplification circuit used for power amplification of an audio signal or other signal and, more particularly, to an amplification circuit with improved efficiency and with improved response characteristic to a sharp rise of a large amplitude input.
As a power amplification circuit for audio signals, an SEPP (single ended push-pull) circuit as shown in FIG. 2 is generally employed. In this circuit, transistors Q1 and Q2 constitute a push-pull circuit in which a signal from a prior stage is received to bases of these transistors and the input signal is amplified by a power supplied from power sources 12 and 14 to drive a load (e.g.,loudspeaker) 10.
Since, in this circuit, the transistors Q1 and Q2 are driven by source voltages +B and -B from which a maximum output can always be provided, this circuit has the disadvantage that power loss (amount of heat generation) in the transistors Q1 and Q2 is large even when an input signal of a small level is applied.
For solving this problem, there has been proposed a voltage switching system as shown in FIG. 3. In this system, source voltage used is switched depending upon the magnitude of an input signal. Power sources 12 and 14 of FIG. 2 are respectively divided in two power sources 16, 18 and 20, 22 (B=B1+B2) and the source voltage of transistors Q1 and Q2 is switched by switches 24 and 26 and diodes D1 and D2. More specifically, when the input signal level is small, the switches 24 and 26 are turned off and the transistors Q1 and Q2 are driven by the source voltages +B1 and -B1. When the input signal level has increased and therefore the source voltages +B1 and -B1 alone cannot supply necessary power to the load 10 any more, the switches 24 and 26 are turned on to drive the transistors Q1 and Q2 by source voltages (+B1)+(+B2) and (-B1)+(-B2).
Since the transistors Q1 and Q2 in this circuit are driven by low voltages +B1 and -B1 when a signal of a small level is applied, loss during application of a small level signal can be reduced as shown in FIG. 4 in comparison with the circuit of FIG. 2 having no voltage switching function. After turning on the switches 24 and 26, however, loss becomes as large as the circuit of FIG. 2 as shown in FIG. 4. In an audio signal for music, variation in amplitude is generally large and probability of switching is very high even though its average power is small. Improvement of efficiency in a large scale in an amplification circuit therefore is rather difficult.
It is theoretically possible to improve efficiency by increasing, as shown in FIG. 5, the number of voltage switching stages, i.e., the number of division of the power source +B. This structure, however, requires increase in power sources, switches and diodes with resulting complex and large scale circuit design. Moreover, in actual circuit design, power loss due to passing through many switches increases and, the provision of the voltage switching stages will be limited to two or three stages as a proper number of stages and, in this case, a large scale improvement in efficiency will be difficult.
There has been used a PWM (pulse width modulation) amplifier as shown in FIG. 6 for improving efficiency. An input such as an audio signal from a preamplifier is applied to a PWM circuit 28 and a PWM signal having duty factors corresponding to signal levels at respective time points of the input audio signal waveform is produced. Push-pull transistors Q1 and Q2 in the output stage are alternately switched by this PWM signal to supply power from the power sources 12 and 14 to a load 10. According to this construction, the transistors Q1 and Q2 are switching-driven and therefore loss during a large output becomes small and efficiency thereby can be greatly improved.
There is another conventional power amplification circuit designed for improving efficiency as shown in FIG. 7. In this circuit, a power amplifier 32 is driven by voltages dropped from source voltages +B (12) and -(14) by using switching series regulators 34 and 36 and loss in the power amplifier 32 thereby is reduced. The switching series regulators 34 and 36 drop the source voltages +B and -B to voltages which are sufficient and necessary for taking out necessary power from the power amplifier 32, by variably switching on-off ratio in accordance with the output (or input) of the power amplifier 32. According to this circuit, as shown in FIG. 8, output voltages of the switching series regulators 34 and 36 change in accordance with the output voltage of the power amplifier 32, so that loss of an output transistor in the power amplifier 32 becomes substantially of a constant value regardless of magnitude of the output of the power amplifier 32. Moreover, since the switching series regulators 34 and 36 are driven in switch-driving, loss in the switching series regulators 34 and 36 is also small and the efficiency as a whole can be greatly improved.
The amplifiers shown in FIGS. 6 and 7, however, have the following drawbacks.
In the PWM amplifier shown in FIG. 6, a filter circuit 30 is inserted between output terminal of the transistors and the load 10 for demodulating a PWM signal provided by the transistors Q1 and Q2 to an audio signal. This causes a speed of rising operation of the amplifier (through rate) to decrease with the result that the amplifier cannot respond sufficiently to a sharp rise characteristic due to a large amplitude of an audio input signal. The speed of response can be improved to some degree by increasing the cutoff frequency of the filter circuit 30 by increasing the switching frequency (frequency of the PWM signal). The increase in the switching frequency, however, increases the number of switching which causes drop in efficiency due to switching loss. This reduces the merit of adopting switching drive. For this reason, with switching frequency within a practical range, the amplifier cannot respond sufficiently to a sharp rise to a large amplitude of an audio input signal.
In the circuit of FIG. 7, the switching series regulators 34 and 36 include a smoothing circuit for smoothing a switching output waveform and supplying the smoothed output to the power amplifier 32. This prevents power supply from responding quickly to a sharp rise due to a large amplitude input with resulting clipping of the output of the power amplifier 32.
It is, therefore, an object of the invention to provide a amplification circuit having an improved efficiency and improved response to a sharp rise characteristic due to a large amplitude of an input signal.